Runtime CPU Spike Detection using Manual and Compiler-Automated Instrumentation Adisak Pochanayon Principal Software Engineer Netherrealm Studios adisak@wbgames.com
Topics to Cover • This talk is about runtime instrumentation-based profiling. • Code Instrumentation Methods • MK’s PET Profiler (Spike Detector)
Profilers • Common types of profilers – Hardware Trace – Hardware Event-based – Software Event-based – Sampling – Instrumented
Manual Instrumentation • Explicit Instrumentation / Code Markup • Wrapper Functions • Detours and Trampolines
Explicit Instrumentation • Requires code markup (source modification) – StartMarker(INFO) / StopMarker(INFO) • Scoped - ScopedMarker(INFO) class CScopedMarker { CScopedMarker(ProfDesc &info) { StartMarker(info); } ~CScopedMarker(ProfDesc &info) { StopMarker(info); } }; #define ScopedMarker(INFO) CScopedMarker(INFO) \ ProfInfo##__LINE__
Wrapper Functions • Compile-time – #define function(…) wrapper_function • Plus – compiler independent • Drawback – only works if you have source code • Link-Time Replacement / Wrapping – GCC option: -Wl,--wrap,function_name __wrap_function_name() __real_function_name()
Wrapper Functions CALLING TARGET FUNCTION FUNCTION
Wrapper Functions 1 2 CALLING “WRAPPER” “REAL” FUNCTION FUNCTION FUNCTION 3 4
Wrapper Functions • Sample of wrapping malloc() with GCC / SNC – Add linker flags: -Wl,--wrap,malloc extern "C" void* __real_malloc(size_t); extern "C" void* __wrap_malloc(size_t Size) { // Call the original malloc() function return __real_malloc(Size); }
Detours and Trampolines • A method of code modification for Instrumentation – Can be done on object code / binaries by Profilers – Run-time with instrumentation library calls • See Microsoft Detours • MIPS Example Code (Handout) • This is another form of manual instrumentation but does not require source markup of the target function.
Detours and Trampolines CALLING TARGET FUNCTION FUNCTION
Detours 1 2 JUMP CALLING DETOUR TARGET FUNCTION FUNCTION FUNCTION 3
Trampolines Trampoline Buffer TARGET PROLOG COPY TARGET CALLING PROLOG TARGET FUNCTION FUNCTION
Trampolines Trampoline Buffer TARGET PROLOG TARGET CALLING PROLOG TARGET FUNCTION FUNCTION JUMP
Trampolines TARGET PROLOG TARGET CALLING PROLOG TARGET FUNCTION FUNCTION JUMP
Detours and Trampolines 1 2 3 JUMP TARGET CALLING DETOUR PROLOG TARGET FUNCTION FUNCTION FUNCTION 4 JUMP 5 6
Detours and Trampolines • Summary ( Drawbacks ) – Roll your own: Trivial on RISC / harder on CISC – Dealing with Page Protection / NX (No Execute) – Commercial implementations are $$$ • Microsoft Detours Software – http://research.microsoft.com/en-us/projects/detours/ • Microsoft 1999 Paper on Detours and Trampolines: – http://research.microsoft.com/pubs/68568/huntusenixnt99.pdf
Manual Instrumentation • Summary of Manual Inst. methods – Explicit Markup – Wrapper Functions – Detours and Trampolines • All require work identifying functions and user intervention (code markup, library calls, or linker parameters).
Automated Instrumentation • Chances are you are already using it – Many profilers support it • Metrowerks CATS, VTune Call Graph, Visual Studio Profiler & Visual C++ /callcap and /fastcap, GNU gprof (w/ gcc – pg) • Compiler-Assisted Instrumentation (CAI) – Allow for User Implementation of profiler but compiler does markup for you • GCC: -finstrument-functions / SNC -Xhooktrace • Visual C++: _penter() & _pexit() using /Gh and /GH
Automated Instrumentation When a compiler generates PROLOG machine code for a function, it generates a prolog (save registers, stack frame, etc.) FUNCTION and epilog (restore saved BODY registers and states prior to return) in addition to the EPILOG function body.
Automated Instrumentation PROLOG Log Entry { _penter() __cyg_profile_func_enter () } FUNCTION Compiler Automated Instrumentation BODY Log Exit { _pexit() __cyg_profile_func_exit () } EPILOG
GCC & SNC CAI • Compiler Option: -finstrument-functions – Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site. void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn,void *call_site);
SNC CAI (PS3 / VITA) void __cyg_profile_func_enter(void *this_fn, void *call_site) { if(0==tls_PET_bIsInProcessing) { tls_PET_bIsInProcessing=true; _internal_PET_LogEntry(0); tls_PET_bIsInProcessing=false; } }
Visual C++ CAI • Using _penter() & _pexit() on Visual C++ is a bit more difficult. – Visual C++ inserts calls on function entry and exit but does not save any registers – At a very minimum, requires writing assembler to save the registers according to the platform ABI – Requires additional checks to be “useable”
Visual C++ CAI – X86 extern "C" void __declspec(naked) _cdecl // Call C Work Function _penter( void ) _internal_PET_LogEntry(0); { tls_PET_bIsInProcessing=false; _asm } { push eax _asm push ebx { push ecx pop esi push edx pop edi push ebp pop ebp push edi pop edx push esi pop ecx } pop ebx pop eax if(0==tls_PET_bIsInProcessing) ret { } tls_PET_bIsInProcessing=true; }
Visual C++ CAI – XBOX 360 • CAI Supported on XBOX 360 (PowerPC) • Almost same as PC 1)Save Registers 2)NEW STEP - Check for DPC (deferred procedure call) 3)TLS – re-entrant check 4)Call actual work function 5)Restore Registers
Visual C++ CAI – XBOX 360 • PowerPC version is more complicated – More ABI Registers to Save and Restore • Have to save and restore FP regs if doing FP – Optimizations and Early Outs – TLS access must be done in ASM • Mixed naked asm / C does not work well like X86 – See handout to follow along…
Visual C++ CAI – XBOX 360 void __declspec(naked) _cdecl __penter( void ) { __asm { // Tiny Prolog // - Set link register (r12) & return address (two steps) std r12,-20h(r1) // Saving LR here is extra step ! mflr r12 stw r12,-8h(r1) // Return Address bl PET_prolog bl _internal_PET_LogEntry b PET_epilog } }
XBOX 360 CAI Flow: _penter() 1 _penter() “C++” {asm} Instrumented Function
XBOX 360 CAI Flow: _penter() 1 2 PET_Prolog 3 {asm} _penter() “C++” {asm} Instrumented Function
XBOX 360 CAI Flow: _penter() 4 1 2 3 PET_Prolog PET_Prolog 3 Early Out {asm} {asm} _penter() “C++” {asm} Instrumented Function
XBOX 360 CAI Flow: _penter() 1 2 PET_Prolog 3 {asm} _penter() 4 “C++” {asm} Instrumented “C++” Profiling Function 5 Routine for Logging Function Entry
XBOX 360 CAI Flow: _penter() 1 2 PET_Prolog 3 {asm} _penter() 4 “C++” {asm} Instrumented “C++” Profiling Function 5 Routine for 6 Logging PET_Epilog Function Entry 7 {asm}
XBOX 360 CAI Flow: _penter() 4 1 2 3 PET_Prolog PET_Prolog 3 Early Out {asm} {asm} _penter() 4 “C++” {asm} Instrumented “C++” Profiling Function 5 Routine for 6 Logging PET_Epilog Function Entry 7 {asm}
Visual C++ CAI – XBOX 360 • PET_Prolog has five sections – Tiny Prolog to save minimal registers – Check for DPC & possible early out – Check for recursion (TLS var) & possible early out – Save temporaries (including r2) and return to parent – Early out returns all the way to grandparent function
Visual C++ CAI – XBOX 360 • Tiny Prolog to save minimal registers // Tiny Prolog // - Save extra registers (r11,r14) // - Set stack frame (r1) std r11,-30h(r1) std r14,-28h(r1) // Old Stack Pointer (r1) is at 0(r1) after this instruction stwu r1,-100h(r1)
Visual C++ CAI – XBOX 360 • Check for DPC & possible early out // Get the TLS thread-specific base lwz r11,0(r13) // Do not try to run in DPC! // In DPC { 0(r13) == 0 } cmplwi cr6,r11,0 beq cr6,label__early_exit_prolog
Visual C++ CAI – XBOX 360 • Check for recursion (TLS var) & possible early out lau r14,_tls_start // Get the TLS global base lau r12,tls_PET_bIsInProcessing lal r14,r14,_tls_start lal r12,r12,tls_PET_bIsInProcessing sub r11,r11,r14 // TLS Base Offset (r11) add r14,r11,r12 // r14 == &tls_PET_bIsInProcessing // Avoid recursion using thread variable tls_PET_bIsInProcessing lwzx r12,r11,r12 cmplwi cr6,r12,0 bne cr6,label__early_exit_prolog li r12,1 stw r12,0(r14) // Set tls_PET_bIsInProcessing
Visual C++ CAI – XBOX 360 • Check for recursion (TLS var) & possible early out That may have looked complicated but it’s actually pretty simple. Here is the C++ equivalent for the last slide: if(tls_PET_bIsInProcessing) goto label__early_exit_prolog; tls_PET_bIsInProcessing=true;
Visual C++ CAI – XBOX 360 • Save temporaries (including r2) and return to parent // Save r0/r2-r10 (temporaries) std r0,8h(r1) std r2,10h(r1) // (r2 is reserved on XBOX 360) std r3,18h(r1) std r4,20h(r1) std r5,28h(r1) std r6,30h(r1) std r7,38h(r1) std r8,40h(r1) std r9,48h(r1) std r10,50h(r1) blr // Return To Caller
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